First off and for the record, there have been books written from multiple viewpoints with two or more characters "writing" in first person. All that is done to differentiate between viewpoints is that the viewpoint character's name is written below the chapter's title on the first page of the chapter. And this is in both "young adult" and mature fiction; I am reading a fantasy book written in first-person from the viewpoint of two half-brothers, and it is turning out quite nicely. However, you probably shouldn't have more than two or three characters looking in on the same set of events; the book I read with about seven viewpoint characters had them actually scattered across the globe in a post-apocalyptic world and, starting out in a couple of groups and occasionally splitting or crossing each other's paths.
As for the "what is preferred" question...well, with first person you can allow the reader to get much closer to a single character, taking away some of the distance between author and reader, and I've found the first-person viewpoint can sometimes make it seem more like someone is telling you their life's story...if it'd done well.
First person is also the preferred viewpoint of amateurs, fanfiction writers, authors of self-inserts and author proxy stories, and people who write in "first person smartass". Unless you know you're good, there's a chance you'll be dismissed based on that alone rather than given a chance to rewrite. It can also make borderline-Mary-Sues into Mary-Sues-From-Hell, since there is great temptation to absolutely go nuts with the story and do *what you want* rather than what would make the story better. A bit of distance is healthy, lest characters change from your pets/projects to your friends/children. It is also usually a lot less subtle than third person.
Personally, I've read great books written in both first and third person. The trilogy of "Idlewild", "Edenborn" and "Everfree" is an example of really well-written first-person narrative, with the first book being entirely from one character's viewpoint and the following two being from multiple viewpoints. "One Flew Over the Cuckoo's Nest" is also written in first person and, in addition to being a great read, is considered a work of literary merit. The "Maximum Ride" series, aimed at younger teens, is a best-seller written in first person, but I find it uncomfortably similar to a lot of self-insert fanfictions due to the way the main (and, unfortunately, viewpoint) character is a wisecracking Mary Sue.
On the Third Person side of the fence, we have quite a few good books as well. Nearly every novelization out there, most modern books considered "literature" and most other published books are written in third person. "House of Stairs" is a short novel written in multiple third person viewpoints that proves just as poignant and even haunting as any first-person story could ever be, and "Ender's Game" isn't far behind it in that respect. While first person tells someone's story, third person makes it one person's story that can also be anybody's story, if that makes sense; Harry Potter would not be as loved by those above the age of twelve if we didn't know and accept that his point of view was not the be-all-to-end-all, and it is the third person POV that allows even the relatively young to make that distinction.
Personally I'd reccomend sticking to third person for longer stories since you don't seem very comfortable with writing and are probably going to get carried away if you start writing "I..." (no offence intended). If there's romance involved for the main character, it's also going to get a bit awkward if you write first person and aren't used to it. Neither POV really limits the viewpoints you can have, but what they do affect is the distance between the author and the reader; unless you're sure the reader will absolutely sympathize with your viewpoint character, third person is the more forgiving option.